Some prior art signal lights comprise a filament lamp, an intermediate screen, and a closure glass or globe. Where appropriate, a mirror may be provided to recover a maximum amount of the light flux which the lamp emits away from the screen and the glass.
Traditionally, the purpose of the intermediate screen is to deflect the light rays received from the lamp (and from the mirror if any) so that the resulting beam that is formed satisfies a given photometric distribution. This must be done in such a manner as, to ensure that the brightness of the light when observed from the outside appears to be as uniform as possible.
It is thus known to provide (preferably concentric) grooves on the inside and/or outside surface of the intermediate screen, with the profiles of respective grooves being designed in such a manner as to provide deflection by refraction or by total reflection that is accurately adapted to the propagation direction of the light rays received from the lamp, and from the mirror, if any.
It will thus be understood that it is extremely fiddly, lengthy, and expensive to make the dies for manufacturing such intermediate screens (which are generally made of molded plastic material which may be transparent or colored). More precisely, it is necessary for each groove or limited group of grooves to adjust the cutting angle of the tool as a function of the prism angles that are to be given to each groove.
In addition, in order to be able to engrave the faces of grooves down to the bottoms of deep grooves between grooves, it is necessary to use a tool which is extremely narrow, and which is consequently fragile.
The present invention seeks to mitigate these drawbacks of the prior art and to provide an intermediate screen which is much easier and cheaper to make, which nevertheless giving satisfactory results, optically speaking.